1. Field of the Invention
The invention is simplified processes for shaping metal, ceramic, and CERMET (ceramic/metallic) parts and shapes from powder/binder mixtures. More particularly, the invention is directed to integration of the compounding and plasticizing processes for economical manufacturing of complex molded shapes which exhibit excellent green strength and which can be readily sintered to produce high quality net shape or near net shape parts.
2. Description of the Related Art
The production of sintered parts from injection molded “green” bodies is well known in the art. Generally, the green body is formed by injecting a fluid powder/binder mixture into a die cavity under pressure and allowing the injected material to cool and solidify in the cavity. The green body, now a self-supporting structure, is then removed from the die cavity and sintered. During the sintering process, the residual binder is volatilized and the powder particles join together to form a monolithic solid body. This process is variously known as Powder Injection Molding (PIM), Metal Injection Molding (MIM), or Ceramic Injection Molding (CIM), depending on the nature of the powdered material.
Heretofore, aqueous compositions useful as injection molding precursors have been pre-compounded, prior to molding or extruding, using batch-type or continuous compounding processes. The prior art teaches that a pre-compounding step prior to molding or extrusion is required in order to achieve an intimate mixture of the binder, solvent and metal and/or ceramic powder, in which the binder material is dissolved in the solvent, and the resulting fluid binder phase is blended with the powder. For agar and agaroid binders, this compounding step has been performed at elevated temperature, since these binder materials are insoluble in water and other solvents at room temperature.
For example, U.S. Pat. No. 4,734,237 discloses the preparation of aqueous metal and ceramic injection molding compounds using a heated sigma blender for blending the precursor liquid and powder ingredients under conditions of time and temperature sufficient to insure solution of the agaroid binder in the fluid carrier phase. Injection molding compounds containing metal powder and agar binder are disclosed in U.S. Pat. No. 5,258,155, wherein the metal powder is kneaded and heated with binder, water and other additives in a batch mixer and then either fed while still hot to the injection molding machine, or cooled and then pelletized for subsequent molding.
Aqueous injection molding compositions containing metal powder are also disclosed in U.S. Pat. No. 4,113,480, wherein the binder and metal powder are pre-blended dry and then combined with a solution of water and additives and mixed sufficiently to dissolve the binder material in the solvent. In this case the binder selected is soluble at room temperature.
U.S. Pat. No. 6,261,496 discloses continuous compounding of aqueous metal and/or ceramic injection molding feedstocks, wherein metal and/or ceramic powders, binder, and liquid carrier are supplied in a continuous manner to a twin screw extruder and then compounded at a temperature sufficient to insure solution of the agaroid binder, extruded, cooled, and pelletized to produce feedstock for subsequent molding.
Pre-compounding of metal and CERMET injection molding and extrusion materials adds significant cycle time, capital equipment requirements, and labor cost to the overall part manufacturing process. Equipment used for either batch or continuous compounding of injection molding materials is intrinsically expensive for several reasons. First, the high viscosities of typical molding materials mandate very rugged mixer design and construction, as exemplified by sigma blade mixers and twin screw extruders. Secondly, heating means and temperature controllers are required. Lastly, continuous compounding requires expensive precision feeders for metering powder and liquid ingredients.
Moreover, in batch and continuous compounding of aqueous binder compositions at elevated temperatures, rapid loss of water by evaporation leads to difficulty in the precise control of the moisture content of the finished product. This is a serious difficulty since moisture content exerts profound influences on the flow properties of the material during injection molding, and on the total shrinkage of the part or shape during drying and sintering with direct impacts on mold filling, green part shape retention, and final product dimensions.
Finally, inevitable yield losses associated with the pre-compounding step add further to overall PIM, MIM, or CIM processing costs.
A need exists for simplified aqueous binder metal and CERMET injection molding processes with lower overall costs and improved process control.